Method and arrangement for tracking a medical instrument

ABSTRACT

The invention relates to a method and an arrangement for the intravascular or intracardial navigation of a catheter ( 5 ). Using an X-ray fluoroscopy device ( 1 ), firstly an image database of 2D images is generated, where at the same time as each 2D image (I) is taken the associated heartbeat phase is recorded using an ECG ( 8 ). During the catheter intervention, the position of the catheter ( 5 ) is measured by means of a position measurement unit ( 6 ), and at the same time the ECG and preferably also a signal that is dependent on the breathing movement are recorded. The current spatial position of the catheter ( 5 ) that is measured is then assigned the 2D image of the image database which corresponds in terms of the heartbeat phase and also possibly in terms of the breathing phase, on which image the position of the catheter can be represented.

The invention relates to a method of tracking a medical instrument thatis inserted into the body of a patient, and to an arrangement that issuitable for carrying out the method.

An arrangement and a method of the type mentioned above are known fromDE 199 46 948 A1. In said document, prior to a catheter investigation,an image database having a number of three-dimensional images of aperiodically moving organ of the body, such as for example of the heart,is generated, where a movement signal of the organ of the body isrecorded at the same time as the images are taken. The movement signalmay be, in particular, an electrocardiogram (ECG) and/or a breathingmovement signal. During the subsequent medical intervention, the spatialposition of the instrument, and also of a reference probe, is determinedby a position measurement unit, and at the same time the movement signalis recorded. Using the movement signal, it is then possible for the 3Dimage that corresponds in terms of the movement phase of the organ ofthe body to be selected from the image database. Using the position ofthe reference probe, which is known in this 3D image, it is thenpossible to determine the current spatial position of the instrumentrelative to the 3D image, and to represent it, for example, superposedon the 3D image. One disadvantage of the known method is the highexpenditure associated therewith. For instance, the three-dimensionalimages of the organ of the body first have to be produced using X-raycomputer tomography or magnetic resonance imaging, and this does notonly take a great deal of time but also greatly modifies theconventional working procedures of a catheter laboratory, since thenecessary imaging methods are not available in the catheter laboratoryand thus the taking of the images requires an additional time-intensivestep. Furthermore, the time-intensive 3D imaging methods generally donot permit real-time updating of the 3D image during the intervention.

In the light of this, it is an object of the invention to simplify thetracking of the position of an instrument, so that it can be integratedmore easily with the working procedure of an investigation andpreferably permits real-time updating of the images during theintervention.

This object is achieved by a method having the features of claim 1 andby an arrangement having the features of claim 11. Advantageousrefinements are given in the subclaims.

The method according to the invention for tracking an instrument that isinserted into the body of a patient, such as a catheter or catheter tipfor example, comprises the following steps of:

a) Detection of a movement signal which represents the movement phasesof a periodic internal movement of the body. Significant examples of aperiodic internal movement of the body are the heartbeat and breathing.

b) Generation of 2D images of a patient body volume of interest, andstorage of the 2D images in an image database, where the associatedimaging parameters (e.g. projection direction, etc.) and the associatedmovement phase (in the simplest case directly expressed by the movementsignal) are stored together with each 2D image during the taking of the2D image.

c) Measurement of the spatial position of the instrument and optionallyalso measurement of the spatial position of at least one referenceprobe, where the reference probe can be fitted in particular externallyon the body of the patient and/or on a medical instrument, or can befitted inside the patient using a reference catheter or another medicalinstrument.

d) Selection of at least one 2D image from the image database, which 2Dimage corresponds in terms of its associated movement phase to themovement phase at the time when the spatial position of the instrumentwas determined in step c).

e) Determination of the position of the instrument on the 2D imageselected in step d).

The method has the advantage that the movement of the instrument istracked using 2D images, which are produced anyway during theconventional procedure of medical (catheter) investigations. It istherefore seldom necessary to use additional equipment. On account ofthe simultaneous storage of imaging parameters, movement phases and 2Dimages in the image database, it is possible, during an operativeintervention that is in progress, to represent the current spatialposition of the instrument on that 2D image from the image databasewhich corresponds to the current movement phase. Displacements andchanges in shape of organs of the body on account of periodic internalmovements can in this way be taken into consideration or compensatedfor. Furthermore, movements of the patient as a whole or of the table onwhich the patient is lying can also be taken into consideration orcompensated for.

An electrocardiogram and/or a breathing movement signal that isdependent on the breathing movement of the patient is preferablydetected as a movement signal. Using these signals, it is possible todetect the most significant periodic internal movements in the body of apatient and thus to take these into consideration when determining theposition of an instrument.

The position of the instrument which is determined in step e) ispreferably represented superposed on the 2D images selected in step d).This makes it possible for the investigating physician to track themovement of the instrument directly on the 2D images.

According to a specific refinement of the method, only 2D images from asingle movement phase are made available for selection from the imagedatabase in step d). This may in particular mean that the image databasemay even contain just a single 2D image. In this case, step d) of themethod is reduced to ascertaining whether or not the only “selectable”movement phase corresponds to the movement phase of the current spatialposition of the instrument. If the movement phases correspond to oneanother, the assignment can take place; otherwise, it must not takeplace. In this way, it is possible to reduce the representationfrequency of the superposed representation of the instrument positionand of the selected 2D image, and so an update of the current instrumentposition will only be represented when the movement phase thereofcorresponds to the predefined movement phase of the 2D image. If theheartbeat is used as a basis for the periodic movement, the predefined2D image can for example correspond to the end-systoles of theheartbeat, so that the superposed representation of the instrument onthe 2D image is only refreshed at these points in the cardiac cycle. Thecomputational outlay for the method can thereby be considerably reduced.By always using the same 2D image, a steady-state image background isestablished, and this facilitates visual tracking of the instrument.

According to another development of the method, step b) on the one hand(generation of 2D images for the image database) and steps c), d), e) onthe other hand (measurement of the spatial position of the instrument;selection of at least one corresponding 2D image; determination of theposition in the image) are carried out a number of times and in varyingorder. This means, in particular, that 2D images can be generated evenwhile the operative intervention is in progress, and this ensures thatthe image database which is used is always up-to-date.

Preferably, the image database contains 2D images from variousprojection directions. As a result, it is possible to represent thecurrent spatial position of the instrument on various 2D images inparallel, or to select the best suited 2D image from a number of 2Dimages from a movement phase.

The 2D images are advantageously generated by means of X-radiationand/or ultrasound, so that the apparatus that is conventionally presentduring a catheter investigation can be used to produce them.

According to a development of the method, at least one reference probeis fitted on a movable X-ray device which is provided for generating the2D images. The spatial position of the reference probe is then measured,according to steps b) and c) of the method, in parallel with thegeneration of the 2D images and with the determination of the spatialposition of the instrument. Knowing the spatial position of thereference probe on the X-ray device makes it possible to determine theprojection direction from the location of the X-ray device, without anyadditional apparatus being required. If another reference probe isadditionally used on the patient, the projection direction can also bedetermined with respect to the patient.

Furthermore, using movement models of the body, it is possible tocompensate for the breathing movement on the basis of the measuredbreathing positions.

The invention furthermore relates to an arrangement for tracking aninstrument that is inserted into the body of a patient, which comprisesthe following elements:

a) A device for generating 2D images of a body volume of interest.

b) A unit for determining the set imaging parameters of the device.

c) A signal measurement unit for detecting a movement signal whichrepresents the movement phases of a periodic internal movement of thebody. As mentioned above, the internal movement of the body may inparticular be the heartbeat and/or the breathing.

d) A storage unit for storing an image database of 2D images of the bodyvolume together with the imaging parameters and the movement phaseswhich belong to the respective 2D image.

e) A position measurement unit for measuring the spatial position of theinstrument that is inserted into the body and optionally of at least onereference probe. The position measurement unit may in particularcomprise a transmitter for transmitting (modulated) electromagneticfields and also a receiver for receiving these fields.

f) A control and computation unit for selecting at least one 2D imagefrom the image database, which 2D image corresponds in terms of itsassociated movement phase to the movement phase belonging to the spatialposition of the instrument, and for determining the position of theinstrument on the selected 2D image.

Using the described arrangement, it is possible for the above-describedmethod to be carried out so that its advantages can be achieved. Inparticular, it is important that the arrangement is compatible with adevice such as is present as standard in conventional catheterinvestigations. In order to be able to represent the position of aninstrument on the 2D imaged generated hereby, the arrangement comprisesa unit for determining the set imaging parameters of the device.Knowledge thereof makes it possible subsequently to convert the measuredspatial position of the instrument into a position on the selected 2Dimage.

Preferably, the arrangement is designed such that it is suitable forcarrying out one or more variants of the method of the type describedabove. Thus, the signal measurement unit may in particular have meansfor measuring an electrocardiogram and/or for measuring the breathingmovement of the patient. Furthermore, the device may in particular be anX-ray apparatus.

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted. The single FIGURE schematically shows the components ofan arrangement according to the invention.

Intravascular interventions are conventionally carried out in a catheterlaboratory. Here, the operation field is observed fluoroscopically usingan X-ray apparatus 1 fitted on a C-arm. In coronary vessel disorders, acontrasting agent is conventionally applied locally, in order to showthe vessel profile on the fluoroscopic image. In the course oftherapeutic methods, such as a PTCA (Percutaneous Transluminal CoronaryAngioplasty) for example, a catheter 5 is pushed into the target area,for example a stenosis in the heart of the patient 4, and its positionis in turn monitored using X-ray fluoroscopy. This means that X-rayfluoroscopy is used not only to show the anatomy of the patient but alsoto navigate the catheter to its target area. Therefore, the patient andthe staff require additional doses of X-radiation which serve solely fornavigation.

The arrangement shown in the FIGURE allows, using the existing imagingapparatus of a catheter laboratory, navigation of the catheter 5 duringcardiac or other intravascular interventions with a reduced amount ofX-radiation. The overall system consists of

a position measurement unit 6, such as for example an electromagneticposition measurement system (cf. DE 199 46 948 A1), which allowsdetermination of the spatial position of the tip of the catheter 5 andalso of a reference probe 3 on the patient 4 or of a reference probe 2on the X-ray device 1 in a stationary coordinate system;

a medical workstation (computer) 7, which receives the positions rdetermined by the position measurement unit 6, the electrocardiogram ECGfrom the ECG system 8, and the X-ray images I from the X-ray device 1;

an X-ray fluoroscopy device 1 which is tracked and calibrated in termsof its setting by a reference probe 2;

a tracked catheter 5 or guidewire;

reference probes 3 on or in the patient;

an ECG system 8 for taking an electrocardiogram using electrodes 9attached to the patient.

Using such an arrangement, it is possible to navigate the catheter 5 inaccordance with the following steps:

1. Preoperative Calibration Phase:

During a calibration procedure, the imaging parameters of thefluoroscopic system 1 are determined. This calibration need be carriedout only once, for example at the manufacturer's premises or duringinstallation of the system 1 (see, for example, U.S. Pat. No. 6,379,043,U.S. Pat. No. 6,471,399).

2. Image Generation Phase During the Intervention:

2.1. Preparation: Preparation of the patient 4 including attachment ofthe ECG electrodes 9; fitting of the reference probes 3 for positiontracking on the patient 4 (in the case of cardiac interventions on thethorax).

2.2. Data gathering:

2.2.1. An X-ray image I or an image sequence is generated using theX-ray fluoroscopy system 1, where the anatomy of interest of the patient4 is visible on the images.

2.2.2. For each image of the taken image sequence, additionalinformation is recorded at the time it was taken, specifically, inparticular, the current position of the C-arm of the X-ray fluoroscopysystem 1, the current position of the reference probe(s) 3 on thepatient and the associated ECG phase. The images and the aforesaidadditional information form multimodal data.

2.3. Data transmission: All images I and the corresponding data aretransmitted to the medical workstation 7. Thus, all information requiredfor navigation is present on the workstation 7.

2.4. If desired or necessary, the imaging steps 2.2. and 2.3. can berepeated for other orientations of the C-arm of the X-ray device 1.

3. Navigation During the Intervention:

During the intervention, the position-tracked catheter 5 or guidewirecan be navigated without the need for further X-ray images:

3.1. Position measurement: First, the position of the patient 4 and ofthe catheter 5 is measured using the position measurement system 6.

3.2. Data gathering: Simultaneously, the ECG phase is measured using theECG system 8.

3.3. Image selection: Based on the ECG phase, the corresponding image orthe corresponding images from the image database obtained in 2.2. areselected by the workstation 7.

3.4. Graphic superposition: From the available data together with theposition of the X-ray device 1, a virtual graphic superposition of thecatheter on the selected X-ray image is carried out during the taking ofthe images, without further images having to be taken by X-ray.

3.5. Repetition: Steps 3.1.-3.4. are carried out continuously.

The arrangement as shown in the FIGURE thus allows very preciseintravascular or intracardial navigation on X-ray fluoroscopy imageswith a reduced amount of X-radiation.

According to a variant of the method, the latter can be carried out witha reduced update frequency. In this case, a reference image is selectedfrom the multimodal data generated in step 2.2. For example, this may bethe image from the end systolic phase of the cardiac cycle. Instead ofagain and again displaying images corresponding to the current ECG phasein steps 3.3. and 3.4., only the aforesaid reference image is used. Thatis to say that an update of the catheter position in step 3.4. on thereference image is only carried out when the currently measuredheartbeat phase corresponds to the heartbeat phase of the referenceimage (that is to say to the end systolic phase). In this way, theupdate rate is reduced to one update per cardiac cycle.

According to another development of the method, in step 2.2.2.additionally a breathing sensor is used, in order to measure the currentbreathing phase of the patient 4. The breathing phase is then storedalong with the images I and the ECG phase in the multimodal data. Instep 3.2., the breathing phase is likewise measured. In step 3.3., theassociated image is then selected on the basis both of the ECG phase andof the breathing phase. The rest of the method then proceeds unchanged.If in phase 2.2. of data gathering not enough data can be obtained, thensuitable interpolation methods can be used in order to calculate thesuperposed catheter position of the image. Furthermore, a movementcorrection for the breathing movement can be carried out as a result ofthe fact that the breathing-induced movement of the heart can becompensated for based on the measured breathing position using amovement model of the heart.

Another development of the method comprises mixing the steps of datagathering (2.2.-2.4.) and navigation (3.1.-3.5.) with one another. Inthis way, it is possible for the fundamental 2D images of the imagedatabase to be verified, in whole or in part, in real-time during theintervention, so that the image database that is used can be regularlyrefreshed in whole or in part.

1. A method of tracking an instrument that is inserted into the body ofa patient (4), comprising the steps of: a) detection of a movementsignal which represents the movement phases of a periodic internalmovement of the body; b) generation of 2D images of a body volume ofinterest, and storage thereof in an image database together with theassociated imaging parameters and the associated movement phase; c)measurement of the spatial position of the instrument; d) selection ofat least one 2D image from the image database, which 2D imagecorresponds in terms of its associated movement phase to the movementphase belonging to the measured spatial position of the instrument; e)determination of the position of the instrument on the selected 2Dimage.
 2. A method as claimed in claim 1, wherein an electrocardiogramand/or a breathing movement signal that is dependent on the breathingmovement of the patient is detected as movement signal.
 3. A method asclaimed in claim 1, wherein the position of the instrument isrepresented superposed on the selected 2D images.
 4. A method as claimedin claim 1, wherein, in step d), only 2D images from a single movementphase are available for selection from the image database.
 5. A methodas claimed in claim 1, wherein steps b) and c) to e) are carried out anumber of times and in varying order.
 6. A method as claimed in claim 1,wherein the image database contains 2D images from various projectiondirections.
 7. A method as claimed in claim 1, wherein the 2D images aregenerated in step b) by means of X-radiation and/or ultrasound.
 8. Amethod as claimed in claim 1, wherein at least one reference probe isfitted on a movable X-ray device which is provided for generating the 2Dimage.
 9. A method as claimed in claim 1, wherein at least one referenceprobe is arranged on or in the body of the patient.
 10. A method asclaimed in claim 1, wherein the breathing movement is compensated forusing movement models of the body.
 11. An arrangement for tracking aninstrument that is inserted into the body of a patient, comprising: a) adevice for generating 2D images of a body volume of interest; b) a unitfor determining a set imaging parameters of the device; c) a signalmeasurement unit for detecting a movement signal which representsmovement phases of a periodic internal movement of the body; d) astorage unit for storing an image database of 2D images of the bodyvolume together with the associated imaging parameters and theassociated movement phases; e) a position measurement unit fordetermining the spatial position of the instrument that is inserted intothe body; d) a control and computation unit for selecting at least one2D image from the image database, which 2D image corresponds in terms ofits associated movement phase to the movement phase belonging to thespatial position of the instrument, and for determining the position ofthe instrument the selected 2D image.
 12. An arrangement as claimed inclaim 11, wherein it is designed for carrying out a method as claimed inclaim
 1. 13. An instrument tracking system comprising: a) a means forgenerating and storing 2D images of a volume of interest in a body priorto insertion of an instrument into the body; b) a means for measuringmovement phases of a periodic internal movement of the body; c) a meansfor correlating said 2D images with said movement phases; d) a means fortracking the position of the instrument upon insertion into the body; e)a means for selecting a stored 2D image based on real-time measurementof the movement phases; and f) a means for superimposing the position ofthe instrument with the selected, stored 2D image.
 14. The instrumenttracking system of claim 13 wherein the periodic internal movement ofthe body is caused by the cardiac system.
 15. The instrument trackingsystem of claim 13 wherein the periodic internal movement of the body iscaused by the respiratory system.
 16. The instrument tracking system ofclaim 13 wherein the means for measuring movement phases includes anelectrocardiogram.
 17. The instrument tracking system of claim 13further comprising at least one reference probe positioned on at leastone of the means for generating 2D images and the body.